The present invention relates to a molding process for making toroidal manufactured articles from closed-cell, low-density, expanded elastomeric material, comprising the steps of: introducing a semifinished product of raw elastomeric material into an annular semivulcanization chamber provided in a mold; closing the mold; semivulcanizing the semifinished product in the semivulcanization chamber; opening the semivulcanization chamber for causing an expansion and a consequent transferring of the semifinished product to the outside of said chamber.
The present invention also relates to an apparatus for putting the above process into practice, said apparatus being of the type comprising: a first half-mold and a second half-mold which are movable between a closed position in which they are located close to each other for defining a semivulcanization chamber designed to accommodate a toroidal semifinished product made of elastomeric material, and an open condition in which there are spaced apart from each other for enabling the semifinished product to be introduced into and extracted from the semivulcanization chamber.
In greater detail, the invention falls within the technical field of the production of toroidally-shaped manufactured articles to be used as such or as a filling material for hollow items of same shape, such as vehicle tires for example, for which a low density, in the order of 120 kg/m.sup.3 for example, and a closed-cell cellular structure are required.
It is known that for making manufactured articles of elastomeric material it is provided that a predetermined amount of raw elastomeric material be introduced into the cavity of a heated mold, within which mold the material undergoes a blowing process and a simultaneous vulcanization. In greater detail, blowing is caused by appropriate foaming agents that, previously introduced into the blend of the raw elastomeric material, give rise to the generation of gas in the form of small pockets or cells homogeneously distributed in the material mass.
Depending on the degree of the expansion occurred in the vulcanization step, the cells in the expanded material will be "open", that is mutually interconnected, or vice versa "closed", that is separated from one another by thin walls of elastomeric material. More particularly, the larger the expansion undergone by the material during the vulcanization step is, the greater the amount of open cells is, as compared to the closed cells.
From the foregoing it appears that it is presently very difficult to produce manufactured articles of closed-cell expanded material having a density lower than predetermined limits. By adopting the usual techniques according to which the expansion and vulcanization of the semifinished product are completed in a single step within the mold, the production of closed-cell manufactured articles having a lower density than approximately 300 kg/m.sup.3 is practically impossible and obtaining densities lower than approximately 500 kg/m.sup.3 already involves accepting a high percent of open cells in the material.
In order to obviate these limits, which cannot be accepted when the production of toroidal manufactured articles of the above type is involved, in accordance with the production techniques currently in use the raw semifinished product is introduced into the cavity of a mold the sizes of which are slightly bigger than those of the semifinished product and surely smaller than those of the manufactured article to be made. Under this situation the semifinished product is submitted to a semivulcanization step during which the expansion of the gases generated by foaming agents is inhibited in order to enable the material surrounding the individual cells to undergo a sufficient solidification by effect of cross-linking.
The mold is subsequently opened thereby enabling the expansion of the semifinished product that will be afterwards removed from the mold for completion of the vulcanization process in hot air.